1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and particularly to a heat-sensitive recording material which comprises a support, a heat-sensitive recording layer which is provided on the support and contains at least two types of compounds which react with each other by heating imagewise to thereby develop color, and a protective layer, in which the heat-sensitive recording material has good film quality and is excellent in transparency, glossiness, and light-fastness, and images can be written and recorded by a thermal head or an infrared laser with high color-developing sensitivity.
2. Description of the Related Art
Heat-sensitive recording has been recently developing because the device used for the heat-sensitive recording is structured simply, is highly reliable, and does not need complicated maintenance. As a heat-sensitive recording material, conventionally, compound obtained through the reaction of an electron donative colorless dye and an electron acceptive compound, and compound obtained through the reaction of a diazonium salt compound and a coupler are widely known.
In recent years, in order to improve properties including (1) color developing density and color developing sensitivity, (2) fastness of color developer and the like in heat-sensitive recording materials, extensive studies have been carried out. However, when a heat-sensitive recording material is exposed to sunlight or displayed on walls at offices or the like for a long period of time, there have been drawbacks in that background portions of the material become colored through irradiation of light, and image areas may become discolored or faded. In order to improve the discoloring or fading of image areas, various methods have been proposed. However, these methods fail to sufficiently provide the above-described improvements.
On the other hand, in multiple fields such as facsimiles, printers, labels, and the like, demands for a heat-sensitive recording system are increasing. Therefore, it is becoming necessary for heat-sensitive recording paper on which images are written by a heat-sensitive recording head or an infrared laser through imagewise heating to provide higher degrees of writing sensitivity or image quality. At this point, conventionally, in order to provide a heat-sensitive recording material with color-developing sensitivity and light-fastness, as well as film quality at the same time, one of the two types of compound which react with each other through the application of heat has been encapsulated in a microcapsule so as not to perform mutual reaction and color-developing during storage. However, the microcapsules may tend to aggregate as time elapses during the preparation of the solution and the application thereof. Accordingly, it has been difficult to obtain a uniform and high quality image through homogeneous dispersion of the capsule solution. Further, when the other compound which performs the color-developing reaction is mixed as an emulsion, or when a latex dispersion solution is added to the microcapsule in order to improve the film quality of the heat-sensitive recording material, aggregation of the microcapsules has been increasing.
A hydrophobic compound is dissolved in oil or the like, and the resultant solution is added to a coating solution as an emulsion and used, however, separation or composition may occur during dissolving or refrigerating processes, and image quality often deteriorates.
Further, in order to improve the durability of the heat-sensitive recording material, a layer which contains a capsule containing an ultraviolet absorbent precursor has been provided at the outermost layer of the heat-sensitive recording material as a light transmittance adjusting layer. However, even in this case, microcapsule aggregation occurs, which triggers a manufacturing disorder of the heat-sensitive recording material. In this way, it has been desired to provide a heat-sensitive recording material which prevents microcapsule or latex aggregation which is contained in each of the layers of the heat-sensitive recording material, and thereby obtains a high quality image having a uniform hue in each of the layers by uniformly dispersing a capsule dispersion solution or a latex dispersion solution. However, in spite of the layered structure of a protective layer, a heat-sensitive recording layer, and the like, there has been a large problem that, when solid matter or an emulsified dispersion of latex or the like and a microcapsule dispersion solution coexist in the same layer, microcapsules aggregate extremely easily.